Recently, a reduction in size of a semiconductor element has been required in order to satisfy demands for a decrease in costs.
Since, however, the miniaturization of the semiconductor element incurs rises in heat generation density and current density, problems to be stated below are involved in prior-art structures wherein wire bonding connections to the semiconductor element are employed, and wherein heat is radiated by disposing a heat sink on one surface of the semiconductor element.
First, with the wire bonding connections, areas in which wires can be bonded become small due to the miniaturization of the semiconductor element, and a rated current is limited by the wires. This gives rise to such a problem that a large current to flow through the semiconductor element is difficult of attainment.
On the other hand, with the construction wherein the heat is radiated by disposing the heat sink on one surface of the semiconductor element, heat radiativity lowers due to the miniaturization of the semiconductor element. Another problem is that, since the heat generation density of the semiconductor element rises, the temperature thereof rises. Accordingly, the construction adversely affects the thermal fatigue lifetime of bonding wire or solder joints.
In order to solve the above problems, therefore, a structure wherein metal members which serve as electrodes and radiation members are soldered on both the sides of semiconductor elements has been proposed as a structure which unites an enhanced heat radiativity based on dual-side-radiating and electric connections based on soldering.
FIGS. 9A and 9B illustrate the general construction of a semiconductor device of this type, in which FIG. 9A is a schematic plan view, and FIG. 9B is a schematic sectional view taken along line IXB—IXB in FIG. 9A.
A semiconductor device disclosed in, for example, JP-A-2001-156219 or JP-A-2003-110064, the contents of which are incorporated herein by reference, has been proposed as the semiconductor device as shown in FIGS. 9A and 9B.
The semiconductor device shown in FIGS. 9A and 9B includes semiconductor elements 10, 11 which have electrodes on a principal front surface and a principal rear surface, respectively, a first metal member 20 which is joined on the principal rear surface side of the semiconductor elements 10, 11 and which serves as an electrode and a radiation member, and a second metal member 30 which is joined on the principal front surface side of the semiconductor elements 10, 11 and which serves as an electrode and a radiation member, and it has a dual-side-radiating molded structure in which the device is substantially wholly encapsulated with a mold resin 80.
Here in the semiconductor device shown in FIGS. 9A and 9B, the second metal member 30 is joined in a state where a third metal member 40 is interposed between this second metal member 30 and the principal front surface of the semiconductor elements 10, 11. Further, one side of the semiconductor elements 10 is connected to signal terminals 60 made of lead frames or the like, through bonding wires 70 within the resin 80.
Further, the semiconductor elements 10, 11 and the metal members 20, 30, 40 are joined to each other through conductive joint members 50 made of a solder or the like, and the joints between them are electrical and thermal joints.
In the semiconductor device having the dual-side-radiating molded structure, therefore, the semiconductor elements 10, 11 are permitted to radiate heat from both the principal front and rear surfaces and to be electrically led out from both the principal front and rear surfaces. Accordingly, the semiconductor device is effective in enhancing its heat radiativity and heightening its current density.
Meanwhile, in the case of the semiconductor device shown in FIGS. 9A and 9B, a part 90 not in the resin 80 (hereinbelow, the part shall be termed “mounted portion 90”) is set in a metal mold, and the resin 80 is poured into the metal mold so as to fill up this metal mold. That is, the resin (mold resin) 80 is molded by molding.
In the case of filling up the metal mold with the mold resin 80, however, there occurs the manufacturing problem that air bubbles remain within the resin 80 for the following reasons:
In the dual-side-radiating molded structure, the metal members 20, 30 are respectively exposed to the rear and front surfaces of the resin 80. Besides, since the metal members 20, 30 function also as the electrodes, an electric field acts across the exposed parts.
As shown in FIG. 9B, therefore, the resin 80 is formed with a thick-walled portion around the metal members 20, 30, namely, at the outer peripheral part of the resin 80, in order to obtain an interval along the surfaces which is necessary for the electrical insulation between the rear metal member 20 and the front metal member 30. Consequently, a distance (Wa+t+Wb) is secured in design as the sum of individual dimensions Wa, t and Wb indicated in the figure.
In this manner, in the semiconductor device of this type, the outer peripheral part of the resin 80 is made of the thick-walled portion, and the part of the resin 80 lying within the mounted portion 90 is made of a thin-walled portion as compared with the thick-walled portion.
Here, let's consider the molding of the mold resin 80. As shown in FIG. 10, in the thick-walled portion which has a large cross-sectional area as viewed in the flowing direction of the resin 80, the flow of the resin 80 is easy in the molding operation, whereas in the mounted portion 90, its cross-sectional area is small due to the existence of the rear and front metal members 20, 30 and the semiconductor elements 10, 11, and the flow of the resin 80 is hindered.
As shown in FIG. 10, therefore, the filling with the resin 80 becomes unbalanced in such a manner that the resin 80 circulates comparatively fast in the thick-walled portion within the metal mold. Besides, the final filling part of the resin 80 extends inside the mounted portion 90, and the air bubbles remain in the resin 80 within the final filling part inside the mounted portion 90. Such air bubbles affect the withstand voltage characteristic, etc. of the semiconductor device, and become problematic in the reliability thereof.